The present invention relates to a device for regulating a turbo compressor unit for supercharging an internal combustion engine, of the type comprising a pressure limiting valve disposed in a branch of the supply duct for the turbine to discharge a part of the exhaust gas upstream of the turbine itself. It is known that in engines supercharged by means of turbo compressors the supply pressure increases with an increase in the speed of the engine. To avoid mechanical and thermal overloads on the engine at high speeds it is therefore necessary to provide means for automatically intervening to limit the supercharging pressure to a predetermined maximum value. The most usual method is to arrange a pressure limiting valve or "waste-gate" valve upstream of the turbine, such valve being operable to discharge upstream of the turbine a part of the exhaust gas directed towards the turbine from the engine. The intervention of the "waste-gate" valve can be controlled either by the supercharging pressure or by the turbine supply pressure; both act on a membrane which causes opening of the valve against the action of a calibrated spring, upon achievement of the predetermined pressure.
The system described is not free from disadvantages. In particular, in the case of a "waste-gate" controlled by the supercharging pressure, the system presents a high back-pressure at the exhaust, which increases with the speed of the engine, and which causes the disadvantage of a high fuel consumption. In the case of a limiting valve controlled by the turbine supply pressure, on the other hand, there is the dual disadvantage of a reduction in the supercharging pressure at high speeds and an inability of the turbo compressor to intervene during rapid accelerations during which a high supercharging would be very useful. To overcome, in part, such disadvantage a parallel control system or "overboost" of the limiting valve has already been proposed, which excludes the valve itself during accelerations by an electrically controlled valve operated by a switch and a timer connected to the accelerator pedal.